All-fiber microfluidic multimode Mach–Zehnder interferometers as high sensitivity refractive index sensors

We present microfluidic multimode fiber Mach–Zehnder interferometers (MMF-MZIs), suitable for refractive index (RI) measurement. The compact and low-cost all-fiber MMF-MZIs were made by forming two coupling points in series at a localized short section (about 1–5 cm) of a multimode optical fiber, using an electric-arc system, thus allowing the excitation and recombination of core and cladding modes. The MMF-MZIs have been integrated with a home-made microfluidic chip, used as a high sensitivity RI sensor to ascertain the surrounding RI variations. This type of microfluidic lab-on-a-chip MZI-based sensor was shown to be effective in sensing very small quantities of samples and its transmitted light signal could easily be measured. The spectra of all MMF-MZIs were Fourier transformed to obtain and the spatial frequency spectra. The MMF-MZIs exhibited many dominant spatial frequencies for all interferometer lengths and interference was mainly constructed by the core mode and dominant lower order cladding modes when the sensing length was 2–5 cm as well as distributed by the core mode and dominant higher order cladding modes when the sensing length was 1 cm. The transmitted spectra of the MMF-MZIs exhibited various sinusoidal-like interference fringes’ amplitudes and peaks changed differently by immersing the MMF-MZIs in different concentrations of sucrose solutions. The microfluidic MMF-MZI sensors have a high resolution of 3.1 × 10⁻⁵ to 7.5 × 10⁻⁵ RIU and a high sensitivity of 160–347 dBm/RIU within RI range from 1.333 to 1.370.